When manufacturing silicon single crystal by the Czochralski method (CZ method), a vitreous silica crucible is used (for example, see Patent Literature 1). In the CZ method, silicon material is put into the vitreous silica crucible, and thereafter heated and fused. Then, a seed crystal is dipped into this silicon melt. The seed crystal is pulled up gradually while rotating the crucible so as to let the single crystal grow. In order to manufacture high-quality silicon single crystal used for semiconductor devices at low cost, it is necessary to raise the yield of silicon single crystal in one pulling process. For that purpose, it is necessary to use a crucible with large capacity which can contain a large amount of material.
In a preparation step before pulling up silicon single crystal in the CZ method, silicon material is pre-filled into the vitreous silica crucible. Such filling operation is mainly conducted by manpower. This is because the vitreous silica crucible is very fragile and tends to easily have cracking or chipping, and therefore cracks easily when loading a large amount of material vigorously. Also, in order to obtain a silicon single crystal ingot as big as possible in one pulling process, it is necessary to try to tightly fill the crucible with a large amount of material at the beginning. For this purpose, it is necessary to carefully conduct the filling operation by taking into consideration the size and shape of small fragments of polycrystalline silicon or the like.
However, although the filling operation is carefully conducted by manpower, there still may be a phenomenon that the vitreous silica crucible suddenly cracks while filling the crucible with the polycrystalline silicon pieces. This phenomenon is considered to be caused by distortion remaining in the silica glass when manufacturing the vitreous silica crucible. That is to say, it is presumed that, when the polycrystalline silicon pieces are put on a position where the residual distortion is significant, even with a weak shock, the shock would be a trigger to cause the vitreous silica crucible's cracking. The vitreous silica crucible's residual distortion is very obvious especially in an annealed crucible formed by annealing a crucible once completed, and in a so-called re-arc crucible formed by repeating arc fusing of a crucible once completed (see patent Literature 2).
If a vitreous silica crucible has cracked, not only the crucible itself cannot be used, but the entire filling operation to fill the crucible with the material conducted until the cracking will be wasted. This will become a quite big loss on the aspect of costs or man-hours. It is difficult to prevent such cracking completely but it is possible to solve this problem by dealing with it as a defective product or a B-grade product when the distortion remaining in a vitreous silica crucible is big. For this purpose, it is necessary to accurately measure the distribution of distortion remaining in a vitreous silica crucible.